SmartCut™ enables a fine layer of a donor substrate to be transferred to a receiver substrate and initiates the following stages:    a) Implantation of atomic species within the donor substrate, in order to create an embrittlement zone of the same depth as the thickness of the fine layer to be transferred.    b) Bringing into contact of the substrates and direct wafer bonding. The entire surface of the substrates is bonded, except for the periphery, as the wafers exhibit edge roll-off (ERO) and are not therefore in contact around their periphery, as can be seen in FIG. 1.    c) Detachment of the donor substrate in the embrittlement zone and transfer of the fine layer to the receiver substrate.
The substrates generally occur in the form of circular wafers; for example, 300 mm wafers are currently in use.
The peripheral area in which transfer of the fine layer has not taken place is called the crown. In relation to FIG. 2, which shows a top view of four peripheral areas of an SOI wafer, the crown CP is located between the perimeter 100 of the receiver substrate (outer edge of the crown) and the perimeter 200 of the transferred layer (inner edge). On the wafer in FIG. 2, the crown CP is even, in other words the SOI perimeter is even.
Prior to direct wafer bonding, it is possible to bring about plasma activation of the surface of at least one of the substrates. This activation enables the bonding strength to be improved significantly.
The bonding strength can also be improved by adapted cleaning prior to bonding, for example a type O3/SC1/SC2 sequence in which the SC1 is carried out at a temperature of under 50° C.
However, it has been found that improving the bonding strength can produce an uneven crown width (referred to in the jargon as a “jagged edge”) in the final product, in other words, the SOI obtained following detachment. What happens here is that following detachment, the crown also includes small isolated zones that have also been transferred.
The width of the crown is therefore made uneven by the random, local enlargement of the transferred zone over several hundreds of micrometres towards the edge of the receiver substrate. The enlargement of the transferred zone is visible in the photos in FIG. 3, where the lighter area represents the transferred layer and the darker layer is the crown.
Document WO2007/06145 discloses a method for preventing the formation of edge voids in an SOI, which are non-transferred zones located preferentially at the periphery of the SOI.
One of the aims of the invention is therefore to improve the method of producing a structure by layer transfer, so as to guarantee that the edge of the transferred layer is even, without any local enlargement.